illuminated by the sun or moon.
Figure 16.4: Cirrostratus clouds are so thin they are sometimes invisible unless backlit by
the Sun or Moon. ( 32 )
Middle clouds may be made of water droplets, ice crystals or both, depending on the air
temperatures. Altocumulusclouds appear as white to gray, puffy stripes rolling across
the sky (Figure16.5). These clouds often occur before thunderstorms. Thick and broad
altostratusclouds are gray or blue-gray. They often cover the entire sky and usually mean
a large storm, bearing a lot of precipitation is coming.
Low clouds usually hold droplets of liquid water, although they may also contain ice when
temperatures are very cold.Stratusclouds are gray sheets that cover the entire sky (Figure
16.6). These clouds may produce a steady drizzle or mist, but do not carry hard rain.
Nimbostratusclouds are thick and dark. They bring steady rain or snow.Stratocumulus
clouds are rows of large, low puffs that may be white or gray. These clouds rarely bring
precipitation. but
Cloudsgrowverticallywhenstrongaircurrentsarerisingupward.Cumuluscloudsresemble
white or light gray cotton and have towering tops (Figure16.7). On fair days, cumulus
clouds may grow upward but produce no precipitation. On hot summer afternoons, though,
cumulus clouds may mushroom into a form that looks like a head of cauliflower. These
clouds may produce light showers.
If the vertical air currents are strong, a cumulus cloud will grow upward until it develops
into acumulonimbuscloud (Figure16.8). Tall, dark and ominous cumulonimbus clouds
are associated with lightning and intense thunderstorms.